How can I get the hotel check-in certificate required for a Japanese visa?

You should be talking about the proof of accommodation required for a three-year multiple-entry tourist visa, which can be obtained very easily.

Stay at the place where you apply for a visa for one night and let the hotel write you one.

Japanese visa is here, don't try to fake it. Once found by the consulate, multiple visas are invalid. And be blacklisted, so you don't have to want to travel to Japan anymore. The Japanese consulate will make random checks and call the hotel to confirm whether the guests are staying. If you don't live, then both the guest and the contracting agency will be punished.

The guest's punishment is to void your visa. The travel agency deducted points and stopped signing.